832 research outputs found
Fermi gamma-ray `bubbles' from stochastic acceleration of electrons
Gamma-ray data from Fermi-LAT reveal a bi-lobular structure extending up to
50 degrees above and below the galactic centre, which presumably originated in
some form of energy release there less than a few million years ago. It has
been argued that the gamma-rays arise from hadronic interactions of high energy
cosmic rays which are advected out by a strong wind, or from inverse-Compton
scattering of relativistic electrons accelerated at plasma shocks present in
the bubbles. We explore the alternative possibility that the relativistic
electrons are undergoing stochastic 2nd-order Fermi acceleration by plasma wave
turbulence through the entire volume of the bubbles. The observed gamma-ray
spectral shape is then explained naturally by the resulting hard electron
spectrum and inverse Compton losses. Rather than a constant volume emissivity
as in other models, we predict a nearly constant surface brightness, and
reproduce the observed sharp edges of the bubbles.Comment: 4 pages, 4 figures; REVTeX4-1; discussion amended and one figure
added; to appear in PR
On the mechanism for breaks in the cosmic ray spectrum
The proof of cosmic ray (CR) origin in supernova remnants (SNR) must hinge on
full consistency of the CR acceleration theory with the observations; direct
proof is impossible because of the orbit stochasticity of CR particles. Recent
observations of a number of galactic SNR strongly support the SNR-CR connection
in general and the Fermi mechanism of CR acceleration, in particular. However,
many SNR expand into weakly ionized dense gases, and so a significant revision
of the mechanism is required to fit the data. We argue that strong ion-neutral
collisions in the remnant surrounding lead to the steepening of the energy
spectrum of accelerated particles by \emph{exactly one power}. The spectral
break is caused by a partial evanescence of Alfven waves that confine particles
to the accelerator. The gamma-ray spectrum generated in collisions of the
accelerated protons with the ambient gas is also calculated. Using the recent
Fermi spacecraft observation of the SNR W44 as an example, we demonstrate that
the parent proton spectrum is a classical test particle power law , steepening to at .Comment: APS talk to appear in PoP, 4 figure
On the Structure and Scale of Cosmic Ray Modified Shocks
Strong astrophysical shocks, diffusively accelerating cosmic rays (CR) ought
to develop CR precursors. The length of such precursor is believed to
be set by the ratio of the CR mean free path to the shock speed,
i.e., , which is formally
independent of the CR pressure . However, the X-ray observations of
supernova remnant shocks suggest that the precursor scale may be significantly
shorter than which would question the above estimate unless the
magnetic field is strongly amplified and the gyroradius is strongly
reduced over a short (unresolved) spatial scale. We argue that while the CR
pressure builds up ahead of the shock, the acceleration enters into a strongly
nonlinear phase in which an acoustic instability, driven by the CR pressure
gradient, dominates other instabilities (at least in the case of low
plasma). In this regime the precursor steepens into a strongly nonlinear front
whose size scales with \emph{the CR pressure}as , where is the scale of
the developed acoustic turbulence, and is the ratio of CR to gas
pressure. Since , the precursor scale reduction may be strong
in the case of even a moderate gas heating by the CRs through the acoustic and
(possibly also) the other instabilities driven by the CRs.Comment: EPS 2010 paper, to appear in PPC
Skeletal Morphology of Opius dissitus and Biosteres carbonarius (Hymenoptera: Braconidae), with a Discussion of Terminology
The Braconidae, a family of parasitic wasps, constitute a major taxonomic challenge with an estimated diversity of 40,000 to 120,000 species worldwide, only 18,000 of which have been described to date. The skeletal morphology of braconids is still not adequately understood and the terminology is partly idiosyncratic, despite the fact that anatomical features form the basis for most taxonomic work on the group. To help address this problem, we describe the external skeletal morphology of Opius dissitus Muesebeck 1963 and Biosteres carbonarius Nees 1834, two diverse representatives of one of the least known and most diverse braconid subfamilies, the Opiinae. We review the terminology used to describe skeletal features in the Ichneumonoidea in general and the Opiinae in particular, and identify a list of recommend terms, which are linked to the online Hymenoptera Anatomy Ontology. The morphology of the studied species is illustrated with SEM-micrographs, photos and line drawings. Based on the examined species, we discuss intraspecific and interspecific morphological variation in the Opiinae and point out character complexes that merit further study
Nonlinear Diffusive Shock Acceleration with Magnetic Field Amplification
We introduce a Monte Carlo model of nonlinear diffusive shock acceleration
allowing for the generation of large-amplitude magnetic turbulence. The model
is the first to include strong wave generation, efficient particle acceleration
to relativistic energies in nonrelativistic shocks, and thermal particle
injection in an internally self-consistent manner. We find that the upstream
magnetic field can be amplified by large factors and show that this
amplification depends strongly on the ambient Alfven Mach number. We also show
that in the nonlinear model large increases in the magnetic field do not
necessarily translate into a large increase in the maximum particle momentum a
particular shock can produce, a consequence of high momentum particles
diffusing in the shock precursor where the large amplified field converges to
the low ambient value. To deal with the field growth rate in the regime of
strong fluctuations, we extend to strong turbulence a parameterization that is
consistent with the resonant quasi-linear growth rate in the weak turbulence
limit. We believe our parameterization spans the maximum and minimum range of
the fluctuation growth and, within these limits, we show that the nonlinear
shock structure, acceleration efficiency, and thermal particle injection rates
depend strongly on the yet to be determined details of wave growth in strongly
turbulent fields. The most direct application of our results will be to
estimate magnetic fields amplified by strong cosmic-ray modified shocks in
supernova remnants.Comment: Accepted in ApJ July 2006, typos corrected in this versio
Extraction of the atmospheric neutrino fluxes from experimental event rate data
The precise knowledge of the atmospheric neutrino fluxes is a key ingredient
in the interpretation of the results from any atmospheric neutrino experiment.
In the standard atmospheric neutrino data analysis, these fluxes are
theoretical inputs obtained from sophisticated numerical calculations. In this
contribution we present an alternative approach to the determination of the
atmospheric neutrino fluxes based on the direct extraction from the
experimental data on neutrino event rates. The extraction is achieved by means
of a combination of artificial neural networks as interpolants and Monte Carlo
methods.Comment: 6 pages, 2 figs, to appear in the proceedings of the 2nd
International Conference on Quantum Theories and Renormalization Group in
Gravity and Cosmology, Barcelona, July 200
“Don’t let the trial kill the intervention”: How can researchers and care home teams implement complex intervention trials in care homes?
There is a need for a context-specific sustainable model for conduct of high-quality research in care homes Implementation of research interventions in care homes needs co-design, good relationships, understanding the context and funding A framework can provide the basis to build trust between researchers, care home residents, families and staff
Iron biogeochemistry across marine systems progress from the past decade
Based on an international workshop (Gothenburg, 14–16 May 2008), this review article aims to combine interdisciplinary knowledge from coastal and open ocean research on iron biogeochemistry. The major scientific findings of the past decade are structured into sections on natural and artificial iron fertilization, iron inputs into coastal and estuarine systems, colloidal iron and organic matter, and biological processes. Potential effects of global climate change, particularly ocean acidification, on iron biogeochemistry are discussed. The findings are synthesized into recommendations for future research areas
On the Momentum Diffusion of Radiating Ultrarelativistic Electrons in a Turbulent Magnetic Field
Here we investigate some aspects of stochastic acceleration of
ultrarelativistic electrons by magnetic turbulence. In particular, we discuss
the steady-state energy spectra of particles undergoing momentum diffusion due
to resonant interactions with turbulent MHD modes, taking rigorously into
account direct energy losses connected with different radiative cooling
processes. For the magnetic turbulence we assume a given power spectrum of the
type . In contrast to the previous approaches, however, we
assume a finite range of turbulent wavevectors , consider a variety of
turbulence spectral indexes , and concentrate on the case of a
very inefficient particle escape from the acceleration site. We find that for
different cooling and injection conditions, stochastic acceleration processes
tend to establish a modified ultrarelativistic Maxwellian distribution of
radiating particles, with the high-energy exponential cut-off shaped by the
interplay between cooling and acceleration rates. For example, if the timescale
for the dominant radiative process scales with the electron momentum as
, the resulting electron energy distribution is of the form
, where , and
is the equilibrium momentum defined by the balance between stochastic
acceleration and energy losses timescales. We also discuss in more detail the
synchrotron and inverse-Compton emission spectra produced by such an electron
energy distribution, taking into account Klein-Nishina effects. We point out
that the curvature of the high frequency segments of these spectra, even though
being produced by the same population of electrons, may be substantially
different between the synchrotron and inverse-Compton components.Comment: 42 pages, 14 figures included. Slightly modified version, accepted
for publication in Ap
Seasonal Changes in Titan's Southern Stratosphere
In August 2009 Titan passed through northern spring equinox, and the southern hemisphere passed into fall. Since then, the moon's atmosphere has been closely watched for evidence of the expected seasonal reversal of stratospheric circulation, with increased northern insolation leading to upwelling, and consequent downwelling at southern high latitudes. If the southern winter mirrors the northern winter, this circulation will be traced by increases in short-lived gas species advected downwards from the upper atmosphere to the stratosphere. The Cassini spacecraft in orbit around Saturn carries on board the Composite Infrared Spectrometer (CIRS), which has been actively monitoring the trace gas populations through measurement of the intensity of their infrared emission bands (7-1000 micron). In this presentation we will show fresh evidence from recent CIRS measurements in June 2012, that the shortest-lived and least abundant minor species (C3H4, C4H2, C6H6, HC3N) are indeed increasing dramatically southwards of 50S in the lower stratosphere. Intriguingly, the more stable gases (C2H2, HCN, CO2) have yet to show this trend, and continue to exhibit their 'summer' abundances, decreasing towards the south pole. Possible chemical and dynamical explanations of these results will be discussed , along with the potential of future CIRS measurements to monitor and elucidate these seasonal changes
- …